This conversion is specificly for dual 6volt (12volt) systems that utilize a single wire plug for the high speed lockout. The plug lockout system was used on most dual 6volt Power Wheels with push button shifting.

By simply unplugging the lockout plug, you now have a perfect location to connect another 6 volt battery. This will provide 18 volts in high and 6 volts in low.

Here is a diagram that should help explain it. (wire colors may vary)

It is very important that you follow the diagram exactly since the connections are polarity sensitive.